The present invention is directed toward a method for demodulating high speed asynchronous time division multiplexed signals and, more particularly, toward a method for demodulating high speed frameless asynchronous time division multiplexed signals.
In wireless communication systems utilizing low Earth orbiting satellites, where data packet switching is employed and Time Division Multiplexing is the selected mode of downlink access, it is advantageous to use a small number of broadband carriers in the downlink, as opposed to a large number of narrow band carriers. This means that the data packets destined for a multiplicity of Earthbound user terminals are time multiplexed into a single broadband, high data rate carrier. However, the data demodulation rate at an individual user terminal may be desired to be much smaller than the carrier data rate, also referred to as the bearer data rate, to reduce demodulator complexity and cost. For example, an exemplary bearer data rate may be 450 Mbits/sec, where an exemplary demodulation rate at an individual user terminal may be 2 Mbits/sec.
Also, as modern satellite communication systems are increasingly becoming cellular in character, the satellites, via high gain antennas, are generating narrow beams, also referred to as xe2x80x9cspot beamsxe2x80x9d and creating small cells on the Earth. In a given satellite footprint, or useful field of view of the Earth from the satellite, there may typically be 360 cells. However, unlike conventional terrestrial cellular communication systems, a satellite communication system may not have all 360 downlink beams active at all times; this would require the generation of 360 simultaneous transmit spot beams and would place a great complexity/cost burden on the satellite payload. To mitigate this problem, a satellite communication system may employ cell hopping by transmitting fewer beams than the number of cells in a satellite""s footprint. Typically, the number of hopping beams might be 24 for the above example of 360 cells in a given footprint.
The capacity of a hopping downlink beam to deliver traffic to a cell is directly proportional to its dwell time at the cell: CAPACITY=(BEARER DATA RATE) (DWELL TIME). As shown in FIGS. 1a-b, in conventional, or synchronous, TDM (Time Division Multiplexed) systems, the time domain is divided into fixed-length/fixed-boundary frames 10, which are further subdivided into fixed-length/fixed-boundary slots 12 and sub-slots 14. If cell hopping were employed in a conventional TDM system, cells 1-n would typically be visited with some fixed periodicity with respect to the frame 10, e.g., once per frame (see FIG. 1a), and the dwell period at a cell (Tslot) would typically be synchronized to the slot 12. Within a given slot 12, capacity is allocated to different receivers 1-k by allocating a fixed sub-slot 14 to a given receiver 1-k (see FIG. 1b), with the dwell period at a receiver (Tsubslot) synchronized to the subslot 14. The capacity allocation is made on a demand-assigned basis through a call set-up protocol, which is relatively time consuming and inflexible.
In the conventional, i.e., synchronous, TDM system described above with respect to FIGS. 1a-b, all cells 1-n have the same dwell time Tslot and the cell visits, i.e., slots 12, occur at times known a priori to the receivers 1-k in each cell 1-n. Variations, still within the commonly accepted definition of a synchronous TDM system, may exist as follows: (a) some cells may have different visitation periods than others, e.g., twice a frame or once every two frames; (b) the slot 12 durations may have non-uniform but fixed lengths; or (c) more than one subslot 14 may be allocated to a receiver. The distinguishing feature of a synchronous TDM system is that by acquiring time synchronization to the system clock, a receiver in any cell has accurate knowledge of the time when it will be accessed.
In synchronous TDM systems, once capacity is allocated to a receiver, it cannot be rapidly redeployed. If the receiver does not use the allocated capacity, it is wasted. Accordingly, modern broadband systems offering bandwidth on demand services tend to favor Asynchronous TDM (ATDM) where the dwell times at a cell and the access times to a given receiver can be dynamically changed without notifying the receiver. Thus, ATDM systems alleviate the overhead and delay of call set-up required in a synchronous demand-assigned TDM system.
ATDM systems belong to one of two categories; framed and unframed. As shown in FIGS. 2a-c, in a framed ATDM system, although there is a fixed-length/fixed-boundary frame 10 and slot 12-structure (see FIG. 2a), the downlink beam hops between cells 1-k within a given slot 12 (see FIG. 2b). In framed ATDM, only cells that have packets to be delivered are visited by the beam, and the dwell time (Tburst) at each cell 1-k is just sufficient to deliver the packets destined for that particular cell 1-k. The cells that can potentially be visited in a given slot 12 are predetermined, but not all cells are necessarily visited in every slot 12; a cell being visited only if there are packets to be delivered. Thus, the cells visited in a particular slot 12 comprise a random subset of a fixed set of cells. As a consequence of the random visitation times, the start and end times of an access (Tburst) to a cell are also random. However, the access to a given cell always occurs within a predetermined fixed-length/fixed-boundary slot 12 (Tslot)
Within the access to a particular cell, Tburst, random numbers of packets are transmitted to random numbers of receivers 1-k, or user terminals (FIG. 2c). The flexible bandwidth on-demand feature of ATDM systems stems from the fact that the mean data rate to a given terminal is proportional to the mean number of packets transmitted to the terminal in unit time, with the number of packets transmitted to the terminal capable of being changed dynamically without the cooperation, or prior knowledge, of the receiver. Thus, the data rate, or bandwidth, to a given terminal can increased by simply increasing the number of packets transmitted to the terminal in each access (Trx) and/or increasing the visitation rate to the cell containing the terminal.
One of the limitations of a framed ATDM system, is that the slot length Tslot limits the capacity peak density that can be created on the ground. As the slot length Tslot is fixed, it limits the maximum dwell time Tburst on the busy cells, since the dwell time Tburst on a given cell cannot exceed the slot time Tslot. On the other hand, if the capacity demand on the ground is highly non-uniform, in some slots there will be idle time after all cells in the slot have been visited leading to capacity wastage. Therefore, in order to achieve operational flexibility in creating capacity peak densities, some broadband satellite systems are opting for frameless ATDM, where all time limitations of the frame and slot are eliminated.
FIGS. 3a-b illustrate the xe2x80x9cpoint and shootxe2x80x9d access to user terminals in a frameless ATDM system. Both the cell revisit time Trevisit and the cell dwell time Tcell of the downlink burst are random. Further, there is no fixed association between a hopping beam and a cell, ie., a cell may be visited by any available hopping beam, although this is not explicitly shown in FIG. 3.
In the above-described synchronous TDM and framed ATDM systems, specific beams were assigned to specific cells, However, which beam visits a cell to deliver a packet is not of great importance in terminal design; what is important is the degree of time predictability of the burst, which is tabulated below.
In frameless ATDM systems, a buffer of packets is maintained in the satellite for each cell in its footprint. Packets uplinked to the satellite from the Earth, or forwarded from other satellites in a multi-satellite system with cross-links, are queued in the buffers waiting to be downlinked to specific terminals in known cells. Based on a packet discharge algorithm, the packets are downloaded to Earth-based terminals in a cell at random firing times and with random dwell times. For example, when the satellite buffer is sufficiently close to being full, or when the packets have been held for a predetermined maximum length of time, a beam is pointed to a particular cell and all packets in the buffer directed to that cell are delivered; higher priority packets will be subject to less queuing delay than lower priority packets. By virtue of framelessness, the flexibility of assigning beams to cells is maximized. A key aspect of the discharge algorithm in frameless ATDM systems is at the start and end times of the download, or access Tcell, are completely unconstrained.
Prior art receiver demodulators for frameless ATDM are generally constructed according to one of the following forms: (a) real time demodulation of all data packets in a burst at the bearer data rate, followed by address based selection of the receiver""s own packets; or (b) non-real-time demodulation of the receiver""s own data packets plus some overhead bits at a rate lower than the bearer data rate. Real time demodulation will require the receiver to have a demodulator operating at the bearer data rate. Since the bearer data rate is generally high, e.g., 450 Mbits/sec, a receiver demodulator would be required to have a 450 Mbits/sec demodulator. This sets the complexity and cost of the demodulator at the receiver at a high level.
Non-real-time demodulation requires the receiver to store the received burst at the bearer data rate, and then read the burst out of memory for demodulation at the generally lower end-user data rate. However, if the receiver begins storing signal samples in its memory upon the detection of a burst, since the end time of an access burst in frameless ATDM is indeterminate, bounded possibly by the queue buffer size in the satellite payload, the signal memory in the receiver terminal would have to be at least as large as the queue buffer in the satellite payload, multiplied by the A/D converter resolution which, in a typical implementation, is in the range of 4-8. This would necessitate the use of very large signal storage memories and would be expensive for a low cost, low data rate terminal.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
A method of demodulating a communication signal is provided according to the present invention. The communication signal includes a plurality of pages with each page having a plurality of addresses and corresponding data packets. The demodulating method includes the steps of receiving the communication signal at a user terminal, identifying which of the plurality of pages are destined to the receiving user terminal, identifying which of the plurality of data packets within the identified page are destined to the receiving user terminal, and demodulating only the data packets identified as being destined to the receiving user terminal.
In one aspect of the present invention, each of the plurality of pages is preceded by a page header indicative of page number. The step of identifying which of the plurality of pages are destined to the receiving user terminal includes the step of detecting which of the plurality of page numbers, corresponding to the plurality of pages, matches the receiving user terminal page number.
In another aspect of the present invention, the page header includes an MFSK (Multiple Frequency Shift Keying) signal having a plurality of center frequencies with each page number represented by a unique sequence of MFSK symbols. The step of detecting which of the plurality of page numbers, corresponding to the plurality of pages, matches the receiving user terminal page number includes the step of filtering the communication signal using a set of narrow bandpass filters having center frequencies corresponding to the unique MFSK center frequencies, followed by energy detection.
In another aspect of the present invention, the step of identifying which of the plurality of data packets within the identified page are destined to the receiving user terminal includes the steps of demodulating the plurality of addresses in the identified page, and detecting which of the plurality of addresses within the identified page matches the receiving user terminal address.
In another aspect of the present invention, the demodulating method further includes the steps of writing the identified page to a memory in the receiving user terminal, and reading the identified page from the memory for demodulation.
In another aspect of the present invention, the step of writing the identified page to a memory in the receiving user terminal includes the steps of writing the plurality of addresses in the identified page to a first memory in the receiving user terminal, and writing the plurality of data packets in the identified page to a second memory in the receiving user terminal. The plurality of addresses in the identified page and the data packets identified as being destined to the receiving user terminal are read out of the first and second memories, respectively, for demodulation.
In another aspect of the present invention, the communication signal is transmitted at a first rate, the data packets identified as being destined to the receiving user terminal are demodulated at a second rate, and the plurality of addresses in the identified page are demodulated at a third rate less than the first rate but greater than the second rate.
In another aspect of the present invention, the communication signal is transmitted at a first rate, and the data packets identified as being destined to the receiving user terminal are demodulated at a second rate less than the first rate.
In another aspect of the present invention, the first rate is approximately 450 Mbits/sec, the second rate is approximately 2 Mbits/sec, and the third rate is approximately 5.7 Mbits/sec.
In another aspect of the present invention, the communication signal is transmitted from an Earth-orbiting satellite.
In another aspect of the present invention, the communication signal includes an ATDM (Asynchronous Time Division Multiplexed) signal.
In another aspect of the present invention, the plurality of addresses and corresponding data packets are arranged by grouping the plurality of addresses together followed by the plurality of data packets.
In another aspect of the present invention, the plurality of addresses and corresponding data packets have a one-to-one correspondence.
In another aspect of the present invention, user terminals having low data demodulation rates are grouped into common pages.
In another aspect of the present invention, the page header includes an analog signal. The step of identifying which of the plurality of pages are destined to the receiving user terminal includes the step of detecting a presence of energy in the page header, the detection of energy indicating identification of a page destined to the receiving user terminal.
In another aspect of the present invention, the step of detecting a presence of energy in the page header includes the step of filtering the communication signal using a narrow bandpass filter.
An alternative method of demodulating a communication signal received at a user terminal is provided according to the present invention.
The communication signal includes a plurality of pages, each page having a preamble having a page header indicative of page number followed by a synchronization word, and a plurality of addresses and corresponding data packets. The alternative demodulating method includes the steps of identifying which of the plurality of pages are destined to the receiving user terminal, comparing the synchronization word with a matched filter at the receiving user terminal to confirm page identification, identifying, upon confirmation of page identification, which of the plurality of data packets within the identified page are destined to the receiving user terminal, and demodulating only the data packets identified as being destined to the receiving user terminal.
In one aspect of the alternative form of the present invention, the step of identifying which of the plurality of data packets within the identified page are destined to the receiving user terminal includes the steps of demodulating the plurality of addresses in the identified page, and detecting which of the plurality of addresses within the identified page matches the receiving user terminal address.
In another aspect of the alternative form of the present invention, the demodulating method further includes the steps of writing the identified page to a memory in the receiving user terminal, and reading the identified page from the memory for demodulation.
In another aspect of the alternative form of the present invention, the step of writing the identified page to a memory in the receiving user terminal includes the steps of writing the synchronization word and the plurality of addresses in the identified page to a first memory in the receiving user terminal, and writing the plurality of data packets in the identified page to a second memory in the receiving user terminal. The synchronization word and the plurality of addresses in the identified page and the data packets identified as being destined to the receiving user terminal are read out of the first and second memories, respectively, for matched filtering and demodulation.
It is an object of the present invention to provide a method of demodulation for high speed frameless ATDM packet data while reducing the complexity and the power dissipation of the demodulator at the user terminal.
It is a further object of the present invention to provide a method of demodulation for high speed frameless ATDM packet data while reducing the complexity and the power dissipation of the demodulator at the user terminal so as to approach those of a demodulator that continuously demodulates only its own data.
It is a further object of the present invention to provide a demodulation system capable of demodulating high speed frameless ATDM packet data while maintaining a reduction in the complexity and power dissipation of the demodulator at the user terminal so as to approach those of a demodulator that continuously demodulates only its own data, and not data destined for another user terminal.
It is yet a further object of the present invention to reduce the complexity of a non-real-time demodulator for frameless ATDM packet data.
It is still a further object of the present invention to provide a method of demodulation for high speed frameless ATDM packet data while restricting the size of the signal storage memory at the user terminal and decoupling it from the size of the data buffer in the satellite payload and, consequently, the burst length.
Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims.